Previous attempts to control impact printer noise have been limited to mechanical and not electronic means. The following is a list of United States patents covering the control of printer noise through purely mechanical means such as sound absorbing materials and sound barrier constructions:
U.S. Pat. No. Des. 276,578 ACOUSTICAL ENCLOSURE FOR A PRINTER PA0 U.S. Pat. No. Des. 261,153 ACOUSTICAL HOOD ASSEMBLY FOR A PRINTER APPARATUS PA0 U.S. Pat. No. 4,636,101 LOW-NOISE TYPE IMPACT PRINTER PA0 U.S. Pat. No. 4,620,810 SOUND PROOF DEVICE OF A PRINTER PA0 U.S. Pat. No. 4,279,525 CALCULATOR PRINTER HAVING AN ACOUSTIC NOISE SUPPRESSOR PA0 U.S. Pat. No. 4,197,024 ACOUSTICAL DAMPING FOR PRINTER PA0 U.S. Pat. No. 4,465,390 PRINTER COMPRISING A NOISE-SEALING PAPER-TRANSPORT ROLLER PA0 U.S. Pat. No. 4,677,676 ACTIVE ATTENUATION SYSTEM WITH ON-LINE MODELING OF SPEAKER ERROR PATH AND FEEDBACK PATH PA0 U.S. Pat. No. 4,480,333 METHOD AND APPARATUS FOR ACTIVE SOUND CONTROL PA0 U.S. Pat. No. 4,677,677 ACTIVE SOUND ATTENUATION SYSTEM WITH ON-LINE ADAPTIVE FEEDBACK CANCELLATION PA0 U.S. Pat. No. 4,589,133 ATTENUATION OF SOUND WAVES PA0 U.S. Pat. No. 4,644,581 HEADPHONE WITH SOUND PRESSURE SENSING MEANS PA0 U.S. Pat. No. 4,654,871 METHOD AND APPARATUS FOR REDUCING REPETITIVE NOISE ENTERING THE EAR
The above mentioned patents, however, generally reduce access to the printer operation causing inconvenience to the user or involve moving parts that are subject to wear and failure. The active noise control process of this invention overcomes these problems and achieves a higher theoretical noise reduction than can be obtained by mechanical means.